Can I PWM this VLDO Regulator?

Discussion in 'General Electronics Chat' started by philipm, Apr 26, 2013.

  1. philipm

    Thread Starter Member

    Jun 27, 2012
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    I'm looking at this datasheet for a Linear LT3021 500mA, Low Voltage, Very Low Dropout Linear Regulator.

    I'd like to be able to PWM the SHDN, but I don't know what I should be looking for in the datasheet to let me know if this is possible or not. It will be supplying somewhere between 350 and 500mA in operation (if that makes any difference).

    Appreciate the help.
     
  2. TheComet

    Member

    Mar 11, 2013
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    May I ask, what do you hope to achieve by doing that? Perhaps there's a more efficient solution. Also, what frequency?

    I've never PWM'd a SHDN pin before, so I can't say what exactly will happen.

    TheComet
     
  3. #12

    Expert

    Nov 30, 2010
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    The only thing I see is: Page 8, transient response ~= 25usec.
    I think that means that if you try to go faster than 40KHz the output will be constantly in an overshoot recovering condition.
    So, modulate a lot slower than 40KHz.
     
  4. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    That never works on a linear reg.
     
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  5. #12

    Expert

    Nov 30, 2010
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    Well, obviously it can be shutdown and not shut down. I think the only thing left to discover is the speed limit. If you do that, please report back here and let us know.
     
  6. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    In simulation, it shuts down very slowly, even with no capacitive load. See the attachment.
     
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  7. #12

    Expert

    Nov 30, 2010
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    That looks like the speed limit is 250 Hz.

    Thanks, Ron. Completely answers the OP. (Apparently the simulator knows things that are not on the datasheet.)
     
  8. philipm

    Thread Starter Member

    Jun 27, 2012
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    3
    Many thanks for the replies. I'm looking for a way to efficiently drive a single high power led (350-700mA and 2.1V drop) from a single LiPo cell (3.0-4.2V). I really need to be able to have different brightness levels (current control would be more efficient, but PWM is also ok).

    I came across the attached way to use the LT3021. I was surprised by the very low 0.2V ADJ value so wanted to explore it further. Upon reflection, it may not be efficient enough (0.2V 'lost' through R and >160mV lost through dropout overhead) which is about 80%. There would be further losses through converting the LiPo voltage down to 2.3V (or massive losses if the LiPo was fed as VIN).

    Is there a way to do it with >90% efficiency?
     
  9. #12

    Expert

    Nov 30, 2010
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    80% of what?

    ps, this is a completely different question. Maybe time to start a new thread:
    Want >90% efficiency from 1 LiPo to (1) 700ma LED, Adjustable
     
  10. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    You can probably apply PWM at the ADJ pin, which should have very fast transient response.

    I've tried making SMPS from linear regulators before with some success, although only suitable for curiosity type experiemnts.
     
  11. #12

    Expert

    Nov 30, 2010
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    30Hz to 60Hz is fast enough to switch visible light and make it appear to be continuously on, and that is achievable with this chip. The question now is, what is .36 volts 80% of that causes a question about efficiency.

    .36 volts is 80% of .45 volts, but how did .45 volts get in here?
     
  12. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    LM2731 set up as constant current sink will come pretty close.
     
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